1. Field of the Invention
This invention relates to pump systems for fuel injection systems.
2. Description of the Related Art
Engine exhaust emission regulations are becoming increasingly restrictive. One way to meet emission standards is to precisely control the quantity and timing of the fuel injected into the combustion chamber to match the engine cycle. For certain engine operating conditions, effective injection rate shaping may result in reduced levels of particulates and oxides of nitrogen in the engine exhaust. For example, a pilot injection prior to the main injection event is known to reduce NOx emissions and a high pressure post injection close to the main injection event is known to reduce particulate emissions.
Some existing rate shaping techniques attempt to control injection rates by making various modifications to the injector nozzle assembly. A newer rate shaping technique utilizes a spill or control valve to control pressure build up within the injector and a separate needle control valve to meter and time the different injection events.
Two control valves are costly to make and to maintain. There is a need for a simpler structure that will provide effective high-pressure post injection close to the main injection event.